This invention relates generally to power driven tools, and more specifically it relates to a power driven tool for tightening or loosening fasteners and also having an impact drive and a speed reducing mechanism.
Power driven tools for tightening or loosening fasteners (e.g., nuts and bolts) are known, and power driven tools incorporating impact drives that can intermittently provide an increased amount of torque for tightening or loosening fasteners are common. In a typical tool, a motor and motor shaft rotate an output shaft for turning the fastener. The impact drive is generally positioned between the motor shaft and output shaft to provide the increased torque as necessary. However, because the impact drives of these tools generally operate at the same rate as the motor, they can prematurely wear out before other components of the tool and can possibly leave the tool unusable.
An impact wrench incorporating a ratchet head is disclosed in co-owned U.S. Pat. No. 4,821,611 (Izumisawa). A pneumatic motor rotates a clutch case that coaxially houses an impact drive. Under normal operation, a cam ball fixed within the clutch case engages a finger of an impact clutch and rotates the clutch conjointly with an output shaft for tightening or loosening the fastener. But when frictional resistance of the fastener exceeds the normal torque output of the tool, the cam ball slides under the impact clutch finger and pushes the clutch axially forward along the output shaft. This conjointly moves a pair of hammers forward into registration with a corresponding pair of anvils of the output shaft. The hammers instantaneously impact the anvils and produce an increased amount of torque in the output shaft for overcoming the frictional resistance of the fastener. Immediately following the impact, the hammers retreat axially rearward and when the cam ball makes one full rotation with the clutch case, the impact process repeats.
However, the clutch case and cam ball generally move at a rate equal to the output speed of the motor, which is very high for pneumatic motors. Therefore when the output shaft is unable to turn the fastener, the cam ball repeatedly pushes the impact clutch and hammers axially forward at a similar rate. This often occurs so rapidly that the hammers impact the anvils before corresponding surfaces fully register, or alternatively the hammers completely miss the anvils and fail to produce any additional torque. Moreover, when the frictional resistance of the fastener exceeds the additional torque produced by the hammers, the cam ball and impact clutch may unnecessarily push the hammers into repeated registration with the anvils before an operator can disengage the motor. This can be hard on components of the impact drive (e.g., the cam ball and impact clutch) and may damage them or prematurely wear them out before other components of the wrench.
Co-owned U.S. Pat. No. 5,199,505 (Izumisawa) also discloses an impact wrench. But here, a direct drive socket head is incorporated into the wrench. The impact drive of this wrench is similar to that of U.S. Pat. No. 4,821,611 and effectively provides increased torque to an output shaft when necessary for tightening or loosening a fastener. But as was previously described for the impact wrench of U.S. Pat. No. 4,821,611, the impact drive of this wrench operates at the same rotational speed as the motor and is susceptible to producing excess, unnecessary impacts that can prematurely wear out components of the drive.
Speed reducing mechanisms, such as reduction gears have been used to reduce rotational speed of tool motors. However, these tools tend to be direct drive and do not have the advantages of a ratchet head. Moreover, these tools may use externally connected gears that can be relatively large. This can require the tools to have larger housings that cannot be held in one hand. An impact wrench incorporating a speed reducing mechanism is disclosed in U.S. Pat. No. 4,505,170 (Van Laere). The wrench includes a high speed electric motor (powered by an external electric current source, such as an auto battery) for turning a direct drive head. The speed reducing mechanism is located between the motor and impact drive and is necessary to accommodate the high speeds generated by the motor when it operates the impact drive. (For example, the speed reducing mechanism reduces breaking power on the motor generally caused by the high speed impacts delivered by the impact drive, which can result in lost lever force to the fastener. The output of Van Laere's impact device directly drives the lug and does not obtain any additional mechanical advantage or speed reduction.
Van Laere's impact drive is generally intended for use only for removing severely jammed fasteners (e.g., nuts on auto tires). In particular, Van Laere discloses that an operator can disengage the impact drive for normal, hobby-type work. Accordingly, the impact drive in Van Laere is not intended for continuous use and should not prematurely wear out. Moreover, Van Laere's speed reducing mechanism generally includes externally contacting gears that transfer axial rotation of the motor away from a common axis of the motor shaft and output shaft to reduce the rotational speed. These gears tend to be large in order to adequately reduce the high rotational speed of the motor by an acceptable amount. Therefore, the housing must also be larger.
Therefore, it would be desirable to incorporate an efficient speed reducing mechanism into a power driven tool having an impact drive. This could advantageously control rotational speed of the motor and could thus control the impact rate of the hammers of the impact drive. Accordingly, components of the impact drive could be less prone to damage and wear, and may last longer, while still providing increased torque to the output shaft when necessary.